Primary attention is being devoted to the behavior of grafts which are only incompatible with respect to weak transplantation antigens in mice and rats. Under certain conditions these grafts can evoke an immune response, while under other conditions they may induce unresponsiveness. The basis for these different responses is being studied. More specifically the projects include investigating the behavior of H-Y incompatible grafts (of various types, e.g., skin, parathyroid, islets of Langerhans), as well as those incompatible for putative skin-specific antigens. Studies are also aimed at determining the genetic basis for the expression of, and response to, H-Y antigen in rats. Finally, some attention is also being devoted to elucidating the mode of action of some coat color determinants of the mouse.